Coaxial cable switch



Nov. 28, 1967 D s, CHURCH ET AL 3,355,684

COAXIAL CABLE SWITCH Filed OCC. 3l, 1966 /A/l/ENTORS D. S. CHURCH BV R. W KORDOS ORA/EV United States Patent O 3,355,684 CGAXIAL CABLE SWITCH Donald S. Church, Westerville, and Ronald W. Kurdes,

Columbus, Uhio, assignors to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights,

N .Il a corporation of New York Filed Oct. 31, 1966, Ser. No. 590,897 3 Claims. (Cl. 335-5) This invention relates to coaxial cable switches.

One form of coaxial cable switch uses a reed switch in series with the center conduct-or of a cable. Although this is a relatively simple structure, the isolation provided when the switch is open is insufficient in some uses.

A swi-tching arrangement that provides greater isolation in its open state appears in schematic form in the testing circuit of FIG. 4 of T. Slonczewski Patent No. 2,666,100. This arrangement comprises a pair of singlepole double-throw switches connected in series by a short length of coaxial cable. These switches are operable, in effect, to open the transmission path which is completed through the coaxial cable and insert a circuit under test in the path. As stated in the patent, the short length of cable reduces signal coupling -via switch capacitances around the circuit under test.

A study of the Slonczewski arrangement has indicated that maximum isolation is produced when the intervening cable is `one-quarter of the signal wavelength. Although the isolation thus provided is greater than that provided by the above-mentioned cable switch, this isolation is still insuflicient for some appli-cations. Furthermore, the overall length of this arrangement places another limitation on its use.

An object of the present invention is to increase the isolation provided by coaxial cable switches.

Another object is to reduce the over-all length of relatively high isolation cable switches.

In accordance with the present invention, a lcoaxial cable switch having good open-circuit isolation and good close-dcircuit conduction characteristics is provided by serially connecting at least three spaced switches in the center conductor of a coaxial line. A three-switch embodiment of the invention having an open-circuit isolation equal to the maximum possible with the two-switch Slonczewski arrangement, for example, is approximately one-twelfth the length lof that arrangement. Furthermore, the opencircuit isolation of the three-switch embodiment may be increased by increasing the switch separations. For example, by doubling the separations the level of the leakage voltage is reduced by a fact-or of approximately eight. The interrelationships of switch separations, numbers of switches and isolations produced will be better appreciated from a study of the following description of a specific embodiment. Furthermore, other objects and features of the invention will become apparent from a study of the following description.

The drawing shows a three-switch embodiment of the invention. This embodiment includes an outer conductor formed of tubularly-shaped non-magnetizable metal. Within outer conductor 10 are three spaced dry reed switches 11, 12 and 13. These reed switches are conventional and are described, for example, in Recent Devel- -opments in Bell System Relays-Particularly Sealed Contact and Miniature Relays, by A. C. Keller, Bell System Technical Journal, January 1964, pp. 15-44. Each switch comprises (with reference to switch 12.) a pair of magnetic reeds 14 that are cantilever supported in a hermetically sealed glass envelope 20. The free ends of the reeds overlap but in the absence of a magnetic field do not come into Contact with one another. In the presence of a lmagnetic field, however, the free ends of the reeds pull into contact with one another to form a through circuit. Such magnetic fields are produced by passing a current I0 (by means not shown) through serially connected coils 15, 16 and 17 which are wrappe-d around outer conductor 10 so that they surround switches 11, 12 and 13, respectively. Since the coils are connected in series, the switches are caused to operate simultaneously.

As stated above, switches 11, 12 and 13 are spaced apart with respect to one another. In particular, switches 11 and 12 are interconnected by la coaxial line comprising a center conductor segment 1S and a segment of outer conductor 1li. Similarly, switches 12 and 13 are interconnected by a coaxial line comprising a center conductor segment 19 and a segment of outer conductor 10.

The switch shown in the drawing may be inserted in a coaxial line, for example, by the use of conventional coaxial connectors. The switch also lends itself to the easy and compact construction of switching matrices by mounting a plurality of these switches as crosspoints between a pair of stripline printed circuit boards using, for example, conventional solder connections between the coaxial switches and the stripline structure.

The advantages of the present invention over the prior art may be better understood by considering the following discussion.

Applying conventional lumped constant analysis to a string of open switches (that is, serially connected switches with substantially zero transmission paths between them) produces the following conventional and well-known voltage divider expression:

Tit Vin where ca -.angular frequency,

C=open switch capacitance, k+1=number `of switches and R=the load impedance.

In other words, each switch added to the string reduces the ratio by adding one to the denominator.

We have found that the above expression does not apply when coaxial cables are connected between the switches. In particular, for (k+1) switches with k pieces of identical lossless coaxial cables interconnecting them, the following expression applies:

A=cos d-f-IC for lossless lines where:

:Milug (phase constant) eR=relative dielectric constant of coaxial cable,

lafk-:relative permeability c-onstant of coaxial cable,

c=velocity of light,

d'=distance between contacts from switch to switch,

Zo=characteristic impedance of coaxial cable in ohms and the remaining symbols have the same meanings as in Equation 1.

A better feeling for the results produced by the invention, and in particular the use of expression (2), may he gained by'solving expression (2) for the val-ue of d necessary in the illustrated embodiment to produce the same maximum isolation produced by the Slonczewski arrangeyment (that is, the value when a one-quarter wavelength cable is used in thev Slonczewski arrangement). When this is done, d equals one-hundredths of a wavelength. Since two separations are present, the total switch length is 2d or two-.hundredths of a wavelength. This is a substantial sawing in lenght as it is less than one-twelfth that of the Slonczewski arrangement.

A still better feeling for the results produced by the invention may be obtained by doubling the above value of d and solving expression (2) for the new value of V Out/ Vm. When this is done, i-t will be found that the new value is about one-,eighth that of the old value. In other words, compared with the maximum isolation producible by the Slonczewski switch, the illustrated embodiment of the invention may be designed to produce eight times the isolation within one-'sixth the length. These are substantial improvements in both isolation and length.

The invention has been disclosed and discussed in relation to a three-switch embodiment in which the switches are of the reed type. It should be understood that various other embodiments -may be devised by those skilled in the art without departing from the spirit and scope of the invention. A study of expression (2) will disclose, for example, that for a fixed value of isolation, the overall lengths of cable switches in accordance with the invention will decrease as the number of switches is increased. In use, there will be, of course, practical limitations to the number of switches that it is ldesirable to use. Furthermore, other types of switches may be used. Reed switches having a latching characteristic producedby the setting of remanent material may, for example, be used to eliminate the necessity of providing holding currents.

What is claimed is:

1. A coaxial cable switch comprising an annular sheath of nonmagnetic electrically-conductive material,

a plurality of at least three switches mounted in said sheath with the contacts of each switch not in excess of one-quarter of a wavelength from the contacts of an adjacent switch,

a plurality of electrically-conductive members connected between said switches, respectively, to form a series combination, and

means for simultaneously operating all of said switches.

2.v A coaxial switch in accordance with claim 1: in which said switches are magnetically responsive and said means for operating said switches comprises means for producing magnetic elds.

3. A coaxial switch in accordance with claim 2 in which said magnetically responsive switches are of the reed type and said magnetic field producing means comprises coil means surrounding the outer periphery of said sheath in the vicinity of each of said switches.

References Cited UNITED STATES PATENTS 3,087,125 4/1963 Scholefield 335-5 BERNARD A. GTLHEANY, Primary Examiner.

H. BROOME, Assistant Examiner. 

1. A COAXIAL CABLE SWITCH COMPRISING AN ANNULAR SHEATH OF NONMAGNETIC ELECTRICALLY-CONDUCTIVE MATERIAL, A PLURALITY OF AT LEAST THREE SWITCHES MOUNTED IN SAID SHEATH WITH THE CONTACTS OF EACH SWITCH NOT IN EXCESS OF ONE-QUARTER OF A WAVELENGTH FROM THE CONTACTS OF AN ADJACENT SWITCH, A PLURALITY OF ELECTRICALLY-CONDUCTIVE MEMBERS CONNECTED BETWEEN SAID SWITCHES, RESPECTIVELY, TO FORM A SERIES COMBINATION, AND MEANS FOR SIMULTANEOUSLY OPERATING ALL OF SAID SWITCHES. 